Migrate insertkeys.py to Python3

PEM files are ASCII-encoded, open them as text file (as opposed to
binary). Avoid relying on __del__. Introduce a prologue and epilogue
methods to emit the <policy> tag only once per output.

Test: build plat_mac_permissions.xml on bramble and compare with
      previous version; identical
Test: build product_mac_permissions.xml on bramble and compare with
      previous version; identical
Test: build system_ext_mac_permissions.xml on bramble and compare with
      previous version; identical
Test: build vendor_mac_permissions.xml on bramble and compare with
      previous version; identical
Bug: 200119288
Change-Id: Iced0acf75bff756453918a411aecb9f4ef8f825d
3 files changed