Support removed API members in modular hidden API processing

Previously, the hidden API flags generated for a bootclasspath_fragment
did not include removed API members. That was because it did not supply
a file containing the dex signatures of the removed API members.

The monolithic hidden API processing uses combined-removed-dex which is
the output of a genrule that takes as input the *removed.txt files from
all the APIs and uses metalava to construct the dex signatures file.
This change does the equivalent for the *removed.txt files for the APIs
provided by a bootclasspath_fragment and then passes them to the rule
that generates the final all-flags.csv.

Bug: 179354495
Test: - Update packages/modules/RuntimeI18N to enable hidden API
        processing.
      m out/soong/hiddenapi/hiddenapi-flags.csv
      - Before this change that fails as the flags generated for the
        i18n-bootclasspath-fragment differ from the monolithic flags.
	After this change that passes.
      - Verify that this change does not change any of the monolithic
        hidden API files.
      m com.android.i18n
      - Verify that the apex file before and after this change are byte
        for byte identical.
Change-Id: I6a21edb8a5231666e3f35b2c99a8687f36dd98fd