check-flagged-apis: change internal format

Switch the internal format to represent Symbols to (something close to)
the format described in section 4.3.2 of the JVM spec, i.e.

  com/android/SomeClass/someMethod(II[Ljava/lang/String;)Z

This will make parsing method parameters from api-versions.xml easier,
as that file already uses this format, and converting API signature
files to the same format is less painful than going in the other
direction.

Bug: 334870672
Test: atest --host check-flagged-apis-test
Change-Id: I1e1fb8fe208cd51cce2cc129f5aa1cb495672c16