Add n2 and dashmap to the manifest

dashmap is currently unused, but will be required after merging in
evmar/n2#112

Bug: 318434287
Bug: 328273370
Test: added n2 to build-prebuilts.sh, and ran OUT_DIR=out prebuilts/build-tools/build-prebuilts.sh
Change-Id: Ied5af144b8b2c21b385521be430f9d0313c00d93

default.xml

@@ -142,6 +142,7 @@
   <project path="external/kotlinc" name="platform/external/kotlinc" />
   <project path="external/kotlinx.metadata" name="platform/external/kotlinx.metadata" />
   <project path="external/ktfmt" name="platform/external/ktfmt" />
+  <project path="external/n2" name="platform/external/n2" />
   <project path="external/ninja" name="platform/external/ninja" />
   <project path="external/nsjail" name="platform/external/nsjail" />
   <project path="external/one-true-awk" name="platform/external/one-true-awk" />
@@ -217,6 +218,7 @@
   <project path="external/rust/crates/crossbeam-utils" name="platform/external/rust/crates/crossbeam-utils" groups="pdk" />
   <project path="external/rust/crates/csv" name="platform/external/rust/crates/csv" groups="pdk" />
   <project path="external/rust/crates/csv-core" name="platform/external/rust/crates/csv-core" groups="pdk" />
+  <project path="external/rust/crates/dashmap" name="platform/external/rust/crates/dashmap" groups="pdk" />
   <project path="external/rust/crates/data-encoding" name="platform/external/rust/crates/data-encoding" groups="pdk" />
   <project path="external/rust/crates/der" name="platform/external/rust/crates/der" groups="pdk" />
   <project path="external/rust/crates/der_derive" name="platform/external/rust/crates/der_derive" groups="pdk" />