Prepare release 2.7.5 with packet API (#269)

Adds the following API functions:
- API to get signature key IDs for mobile:
	func (msg *PGPMessage) GetHexSignatureKeyIDsJson() []byte

- API to get encryption key IDs for mobile:
	func (msg *PGPMessage) GetHexEncryptionKeyIDsJson() []byte

- API to get the number of key packets in a PGP message:
	func (msg *PGPSplitMessage) GetNumberOfKeyPackets() (int, error)

- API in package `helper` to encrypt a PGP message to an additional key:
	func EncryptPGPMessageToAdditionalKey(messageToModify *crypto.PGPSplitMessage, keyRing *crypto.KeyRing, additionalKey *crypto.KeyRing) error

.github/workflows/android.yml

@@ -23,6 +23,12 @@ jobs:
         go-version: ^1.16
       id: go
 
+    - name: Install NDK
+      uses: nttld/setup-ndk@v1
+      with:
+        ndk-version: r23c
+        link-to-sdk: true
+
     - name: Checkout
       uses: actions/checkout@v2