[aapt2] Generate @FlaggedApi annotations

Like we currently do for @TestApi and @SystemApi, we look for
"@FlaggedApi" and its parameters in the comment above the resource.

Bug: 295328308
Test: Did the following:
1. Ran `atest aapt2_tests`
2. Ran `aapt2 link` on public-staging.xml with an added @FlaggedApi
   in the comment for the resource currently there. Verified the
   @FlaggedApi annotation is present in the generated R.java.
3. Ran `m framework-minus-apex` and it built successfully.
4. Modified the @FlaggedApi comment with an invalid value (e.g.,
   `Flags.FOO` which does not exist). The build failed. Since
   @FlaggedApi is a source-only annotation (i.e., not present in the
   R.class file), we want to verify it was actually used in the build.
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:6fa7a3c7c5213ad3a94406f46fec6d1219b3fa43)
Merged-In: I0739f888d40269915c0acea5650c386da1a1782e
Change-Id: I0739f888d40269915c0acea5650c386da1a1782e